In the field of telecommunications switches, with increased circuit density and higher switch speed rates (200 Mbits.sup.-1 increasing to 2.4 Gbits.sup.-1 in the future), many proposals have been made for more efficient interconnections of the circuit boards comprising the switch. One area of critical importance is the backplane or motherboard which provides interconnections between the circuit boards. In addition to considerations such as removing heat generated during operation, one important consideration is distribution of clock signals etc to the various boards; at such high bit rates, the problems of propagation delay, bandwidth and crosstalk arise by the clock signals being conducted along conductors in the backplane.
It is known, see K. H. Brenner and F. Sauer "Diffractive-Reflective Optical Interconnects" Applied Optics Oct. 15, 1988, Vol 27, No. 20 to employ hologram elements disposed on a surface to reflect or refract light between transmitters and receivers on an adjacent substrate. Usually the transmitters and receivers form part of an array of integrated circuit chips and the light is employed to distribute signals to the various chips with increased efficiency. The surface carrying the holograms is normally bonded to the chip substrate via a transparent medium to form a compact and monolithic assembly: this is required because of the high accuracy required in alignment of the various optical paths.